Yellowtail kingfish growing up and slowing down

Like most marine fish, yellowtail kingfish have a long way to grow on their own. A 0.0003kg juvenile and 16 kg adult.

In this project I took advantage of the fact that fish produce the smallest free-living vertebrates to test whether the metabolic rate of yellowtail kingfish during growth from an egg to adult changes in the same way as that observed between animal species of different sizes.

For many decades biologists have sought correlations between mass and metabolic rate for a range of animals. The scaling of metabolism with mass is probably best known in the form of Kleiber's law, and describes the general pattern that smaller animals have a higher metabolic rate per-gram-of-tissue than larger animals. An often cited example is that the metabolic rate of a mouse, gram-for-gram, is over 100 times that of an elephant. The scaling exponent which relates metabolic rate to body mass has been, and still is, a matter of debate. To some, this exponent reflects a constraint imposed by the arrangement and scaling of biological systems.

The famous mouse-to-elephant curve from Brody, showing the relationship between metabolic rate of animals of different sizes. Does this relationship imply that an animals' metabolic rate is mainly a product of its size?

There is plenty of data on how mass and metabolic rate correlates between species, but how does metabolic rate scale during the ontogeny (lifetime) of an organism? This question is not easy to answer with mammals, as there is relatively little increase in mass from birth to maturation. With fish, however, it is possible measure the metabolic rate from the moment of fertilization. The difference in mass between a fish egg and adult can be up to a 100 000 fold. This mass range is comparable to the difference between a mouse and an elephant. By measuring the metabolic rate of fish from larvae to adults, there should be adequate statistical power to compare inter- versus intraspecific scaling of metabolic rate. If there is indeed a universal pattern of metabolic scaling with mass, it would be expected that this pattern should also exist within the lifetime of a species, as well as between species. We tested this hypothesis by measuring the oxygen consumption rate of yellowtail kingfish from a larva to an adult, and analysed the data using four proposed scaling models.

From the above figure it was clear that a three of the four of proposed scaling models fitted our data fairly well. The obvious question is which model provides the best fit? There are a number of methods of assessing the strength of statistical correlations, including regression coefficients and information criterion. Below is a table comparing the goodness-of-fit of the different scaling models.

Statistical evaluation of different scaling models used in the graphs above. The regression coefficients (R) aren't particularly useful in separating out the models. The Information Criterion (AIC and SIC) values suggest that the segmented regression and quadratic models are more informative than the other two.

Overall, one would probably argue that there was no definitive answer as to which single model best fits the data. The most accurate concluding statement is that the metabolic rate of yellowtail kingfish probably does not scale linearly during ontogenetic development. As to the question of what this study means for the comparison of interspecific (between species) versus intraspecific (ontogenetic) scaling of metabolism, one can clearly see that the ontogenetic scaling of metabolism in yellowtail kingfish does not follow the widely cited scaling slopes of 0.75 or 0.67.

In summary:

Fish are useful animals to use in comparisons of intraspecific versus interspecifc metabolic scaling because that they are free living from a single cell and grow many orders of magnitude larger.

The regression models used in mass-metabolism scaling studies should be evaluated using a number of goodness-of-fit measures.

The metabolic rate of yellowtail kingfish changes during ontogeny in a non-linear fashion.